The present invention relates generally to circuitry for driving an ignition coil of an electronic ignition system, and more specifically to such circuitry incorporating an ignition coil current limiting feature therein.
In the past few decades, the automotive industry has striven to expand both the number and types of vehicular functions and systems subject to computer control. As an example of one such system subject to computer control, a modern automotive ignition system typically includes an ignition coil and a coil current switching device responsive to an ignition, or "drive", signal to energize the ignition coil. Control circuitry, typically under the direction of a microprocessor-based controller, provides the drive signal to the coil current switching device to thereby energize the primary side of the ignition coil.
Upon initially energizing the ignition coil, the coil current begins to increase, as is common with inductive loads. In accordance with one type of ignition application, known in the industry as a "ramp and hold" application, the current in the coil is held at a constant specified level until the spark plug is instructed to fire. Such a ramp and hold application thus requires an ignition coil current limiting feature to ensure proper operation. Another type of ignition application, known in the industry as a "ramp and fire" application, requires the spark plug to fire when the current in the coil increases to within a particular current range. Although the ramp and fire operation itself does not require a coil current limiting feature, one or more undesirable effects may result under certain conditions unless some type of upper coil current limit is established.
First, the primary side of the ignition coil may saturate at high coil current values, thereby reducing the total energy delivered to the secondary coil and ultimately to the spark plug. Second, an ignition control system error or fault condition may result in the ignition coil being energized for an extended period of time (commonly referred to as an excessive dwell or "on" time). This condition, in turn, results in excessive current flow through the coil current switching device, which may ultimately result in damage to the coil current switching device or to the coil itself. Such a fault condition may occur within either of the foregoing ignition applications.
It is therefore desirable in most ignition applications to provide circuitry for implementing a coil current limiting feature by establishing a maximum current that may flow through the primary ignition coil.
One type of prior art automotive ignition system incorporates the control circuitry and coil current switching device into a single ignition module using a so-called hybrid electronics technology. Essentially, hybrid electronics is an amalgamation of integrated circuit technology and discrete electronic component technology arranged and surface mounted on a ceramic substrate. An example of a portion of one such prior art hybrid ignition control module 10 is shown in FIG. 1. Referring to FIG. 1, ignition control circuitry 12 is connected to a voltage 'source (V.sub.BATT) 14 and provides the voltage V.sub.BATT as VIGN 16 when the engine is running. VIGN 16 is connected to one end of a coil primary 18 which is connected at its opposite end to the drain 20 of power MOSFET 22. The source 24 of MOSFET 22 is connected to one end of a resistor R.sub.SL 28 which is also connected to a voltage sense input 30 of the ignition control circuitry 12. The opposite end of R.sub.SL 28 is connected to a ground reference. The gate 26 of MOSFET 22 is connected to a gate drive output of ignition control circuitry 12. In operation, the gate 26 is provided with a gate drive voltage from ignition control circuitry 12 which turns on MOSFET 22 so that the coil current I.sub.L flows through coil primary 18, MOSFET 22 and R.sub.SL 28. As I.sub.L increases, the voltage across R.sub.SL thus increases, and this voltage increase is monitored at sense voltage input 30 of ignition control circuitry 12. When the voltage across R.sub.SL increases to a predetermined level, the ignition control circuitry decreases the gate drive voltage to the gate 26 such that a relatively constant maximum coil current I.sub.L is maintained.
Designers of automotive ignition modules have recently attempted to provide smaller and more reliable ignition modules by designing so-called "single chip" ignition coil control circuits. Such circuits incorporate the control circuitry and coil driver device into a single high voltage integrated circuit, typically formed of silicon. An example of a portion of one such prior art "single chip" ignition coil driver circuit 50 is shown in FIG. 2. Referring to FIG. 2, input IN 52 is connected to drive control circuitry 54, which is further provided with a voltage source input V.sub.S 56 for receiving a supply voltage thereat. Drive control circuitry 54 also has an output connected to the base of transistor 58 which forms part of a darlington-connected bipolar transistor pair 60. The collector 62 of the transistor pair 60 is connected to one end of an ignition coil primary 64, with the opposite end of coil primary being connected to battery voltage V.sub.BATT 66. The output emitter 68 of transistor pair 60 is connected to one end of a resistor R.sub.SL 70, with the opposite end of R.sub.SL 70 being connected to a ground reference. The output emitter 68 of transistor pair 60 is further connected to one input of an amplifier 72, with another input of amplifier 72 being connected to a reference voltage V.sub.REF 74. The output 76 of amplifier 72 is connected to drive control circuitry 54.
The operation of ignition coil driver circuit 50 is similar in many respects to module 10 of FIG. 1. In circuit 50, a control signal is received at input IN 52 to which drive control circuitry 54 is responsive to drive the base 58 of darlington-connected transistor pair 60. As transistor pair 60 turns on, an increasing load current I.sub.L flows through the coil primary 64, transistor pair 60 and resistor R.sub.SL 70. The voltage drop across R.sub.SL 70 is compared to V.sub.REF 74 by amplifier 72, and the drive control circuitry 54 decreases the drive voltage to the base 58 of transistor pair 60 as the voltage across R.sub.SL 70 approaches that of V.sub.REF. When the load current I.sub.L reaches a predetermined magnitude, drive control circuitry 54 maintains a drive voltage at the base 58 of transistor pair 60 to thereby maintain the predetermined magnitude of I.sub.L flowing through the coil primary 64 for the duration of the control signal. Thus, the coil current limiting feature of circuit 50 is essentially identical to that of module 10, wherein the operation of amplifier 72 represents one known technique for utilizing the voltage drop across a sense resistor R.sub.SL to limit the load current I.sub.L therethrough.
Although the foregoing prior art approaches for limiting ignition coil current have been well received in the automotive industry, they suffer from several inherent drawbacks. First, the current limiting operation depends on the voltage drop across the sense resistor R.sub.SL (28 and 70), which adds to the coil driving device's (22 and 60) saturation voltage. This has the effect of increasing power dissipation and reducing power to the coil primary.
Second, since the load current I.sub.L in the coil primary is typically on the order of tens of amperes, the sense resistor R.sub.SL must be sufficiently robust to dissipate the high power associated with such a load current. In prior art hybrid ignition control modules, discrete sense resistors having 5-10 Watt power ratings have been used to insure safe dissipation of power. However, since the voltage drop across the load current sense resistor is often compared to a reference voltage generated within integrated circuitry, this approach sacrifices accuracy due to inherent mismatches between discrete and integrated circuit components, particularly over the wide temperature ranges typically required of automotive circuitry. Achieving greater accuracy requires resorting to a "single chip" approach wherein the voltage developed across the load current sense resistor and the reference voltage circuitry are each generated "on chip", preferably using matching circuit components. However, "single chip" ignition coil driver circuits have their own inherent disadvantages which are discussed in greater detail in related U.S. Ser. No. 08/522,982, filed Jul. 31, 1995, entitled IGNITION COIL DRIVER MODULE, filed by John R. Shreve et al., and assigned to the assignee of the present invention.
What is therefore needed is an ignition coil driver arrangement having a coil current limiting feature that overcomes the foregoing undesirable characteristics associated with the prior art approaches. Such an ignition coil driver arrangement should be easily and inexpensively produced to form an advantageously compact device which is both reliable and accurate in its coil current limiting feature.